Mechanical APDL Structural Analysis Guide

15.7.3. Problem Sketch

Figure 15.5: Diagram of a Beam With Deformation Indicated

15.7.4. Eigenvalue Buckling and Nonlinear Collapse

Eigenvalue buckling calculation is a linearized calculation, and is generally valid only for elastic structures.
The yielding of materials occurs usually at loads lesser than that predicted by eigenvalue buckling
analysis. This type of analysis tends to need less computation time than a full nonlinear buckling ana-
lysis.

You can also perform a nonlinear load versus deflection study, which employs an arc length solution
strategy to identify critical loads. While the approach is more general, a collapse analysis may be com-
putationally intensive.

The nonlinear collapse analysis must be performed on a structure with imperfections built in to the
model, since a perfect model will not show signs of buckling. You can add imperfections by using ei-
genvectors that result from an eigenvalue buckling analysis. The eigenvector determined is the closest
estimate of the actual mode of buckling. The imperfections added should be small when compared to
a typical thickness of the beam being analyzed. The imperfections remove the sharp discontinuity in
the load-deflection response. It is customary to use one to ten percent of the beam/shell thickness as
the maximum imperfection introduced. The UPGEOM command adds displacements from a previous
analysis and updates the geometry to the deformed configuration.

15.7.5. Set the Analysis Title and Define Model Geometry

1. Choose menu path Utility Menu> File> Change Title.


2. Enter the text "Lateral Torsional Buckling Analysis" and click OK.


3. Start the model creation preprocessor and define the keypoints for the beam. Choose menu path
   Main Menu> Preprocessor> Modeling> Create> Keypoints> In Active CS, and enter these keypoint
   numbers and the coordinates in the dialog as indicated:

Click This Button to

Accept Values

Keypoint X Location Y Location Z Location

Number

1 0 0 0 Apply

2 100.0 0 0 Apply

3 50 5 0 OK

Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information

Beam Analysis and Cross Sections